**3. Results and Discussion**

### *3.1. Material Structure Analysis*

The cross-sections of the material's structure based on 100% HSS and experimental CPHSS samples were studied for comparative analysis. The fine structure of 100% HSS consists of martensite in which carbides of standard alloying elements are distributed (Figure 5a). There is practically no carbide inhomogeneity in the CPHSS with an 80% HSS, 15% TiC, 5% Al2O3 structure (Figure 5b). The pronounced inclusions of refractory compounds TiC and Al2O3 have an irregular shape and size from 0.5 to 4.0 μm. SEM images of structures of other studied CPHSSs are not shown since they turned out to be similar to Figure 5b.

**Figure 5.** SEM images of cross-sections of HS6-5-2C industrial steel (**a**) and experimental CPHSS sample of 80% HSS, 15% TiC, 5% Al2O3 (**b**) (× 1000).

#### *3.2. Testing of Tool Materials under Stationary Loads*

Subsequently, four variants of tool materials were tested under stationary loads (under conditions of abrasion during calostesting). Figure 6 presents experimentally obtained data illustrating the dependence of the volume of worn material on testing time for resistance to abrasive wear; the volume of worn-out material was calculated using the formulas given in [84]. The presented dependences give us a specific idea of the kinetics of the development of abrasive wear of the contact areas of various tool materials over time. The CPHSS samples differ in many respects from the standard HSS, which was expected since the hardness of the latter is lower than that of CPHSS, and the average friction coefficient over steel is higher than that of CPHSS.

**Figure 6.** Dependences of the abrasion of the contact areas of various samples of tool materials on the time of testing for abrasive wear resistance: (1) 100% HSS; (2) 80% HSS, 20% TiC; (3) 80% HSS, 15% TiC, 5% Al2O3; (4) 80% HSS, 20% TiCN.

Figure 7 shows the characteristic 3D profiles of well-shaped wear spots for various samples, measured after 20 min of testing. It can be seen that the lowest abrasion rate is characteristic of the sample containing 80% HSS, 20% TiCN. The sample containing 80% HSS, 15% TiC, 5% Al2O3 had a slightly higher intensity. The minimum wear resistance of all CPHSSs is shown by a sample of 80% HSS, 20% TiC.

**Figure 7.** The 3D profiles of wear centers of various samples of tool materials after 20 min of testing under conditions of force action of a rotating sphere in an abrasive environment: (**a**) 100% HSS; (**b**) 80% HSS, 20% TiC; (**c**) 80% HSS, 15% TiC, 5% Al2O3; (**d**) 80% HSS, 20% TiCN.

The above results are demonstrative but not informative enough since they do not give an idea of the possible adaptive ability of experimental materials and the effectiveness of their use in the manufacture of cutting tools. They require the impact of heat–power loads at a level as close as possible to the operational loads (in machining).

### *3.3. Quantitative Assessment of the Cutting Part Wear and Roughness*

Figure 8 shows the laboratory tests' results of the tool in milling 41CrS4 steel and gives a quantitative assessment of the dimensional wear of the cutting part and the roughness of the processed surface of the workpiece. It can be seen that the curves of the development of wear for CPHSS samples (Figure 8a) over time are of a classical nature, and they mainly show three stages: running-in (I); steady-state wear (II); critical wear (III). At the same time, zone II is noticeably narrowed for a tool made from 100% HSS, and there is no pronounced transition to zone III. Curves of changes in the roughness of the workpiece (Figure 8b) correlate well with the curves of wear development: at the initial stage of testing during running-in (natural blunting) of the tool, no change in roughness is observed; upon transition to the normal wear zone, there is a gradual increase in the contact area of the rear surface of the tool with the surface of the workpiece, accompanied by an increase in the adhesion component of the friction force and a gradual increase in the roughness of both the tool and the workpiece; contact interaction processes are rapidly intensified at the last (critical) stage of wear, and cutting forces increase, which causes a noticeable deterioration in the quality of the surface layer of the workpiece [85]. The tool life is significantly improved for CPHSS compared to a standard tool material and is 17 min for HS6-5-2C and 29–35 min for CPHSS samples. It can be seen that the highest durability (35 min) among the investigated experimental materials is shown by a sample containing 80% HSS and 20% TiCN, and the least (29 min) is shown by the sample containing 80% HSS and 20% TiC. At the same time, it is evident that the increase in resistance achieved for all CPHSS samples can be regarded as a technologically significant result. The results of testing a sample of 80% HSS and 20% TiC (Figure 6, curve 2) showed the maximum increase in the wear of the contact zone after 15 min of milling and by the end of the tests. The volume of abrasion even slightly exceeds the wear of a sample made from traditional HSS. The intensity of material destruction under an abrasive action is largely determined by the ratio of the counterbody hardness and the test metal surface hardness in the contact zone. A sharp increase in the wear rate of an 80% HSS and 20% TiC sample can be associated with a decrease in the hardness of its surface layer at a certain moment. The specific structure of CPHSS (Figure 5), containing a significant number of refractory inclusions of polyhedral and fragmentary shapes, undissolved in the steel bond, suggests that the level of hardness (and the intensity of abrasive wear directly related to it) of such a material is determined by the degree of preservation of the high-hardness carbide phase. It is most likely that over time and with an increase in the volume of abrasion, hard and, at the same time, very brittle TiC particles (their ductility increases at temperatures above 600 ◦C) are not retained in the steel bond and crumble, significantly changing the conditions of contact interaction between the tool material and counterbody. For the other two studied CPHSS samples (Figure 6, curves 3 and 4), a high degree of safety in the steel bond of high-hardness components is ensured throughout the entire test cycle. However, it is too early to conclude the prospects of one or another CPHSS variant based on evaluating the resistance to abrasion without thermal action.

**Figure 8.** Dependences of cutting wear of flank face (**a**) and 41CrS4 steel workpiece's surface roughness (**b**) on the time for various tool materials in milling at *V* = 82 m/min, *f* = 0.15 mm/tooth, *B* = 5 mm, and *t* = 0.5 mm: (1) 100% HSS; (2) 80% HSS, 20% TiC; (3) 80% HSS, 15% TiC, 5% Al2O3; (4) 80% HSS, 20% TiCN.

It is well known that the temperature on its contact surfaces has a decisive influence on the tool life during machining [86,87]. The contact areas of the rubbing surfaces of the tool, chips, and workpiece being processed are small, and the pressure and friction rate exerted on them are extremely high. The high temperature in the cutting zone is the cause

of structural changes in the material of the cutting tool and the reason for the rapid loss of its performance [88–90]. Therefore, to explain the physical nature of the increase in the tool material durability, it is crucial to study the changes in temperature fields in the cutting wedge of the tool during operation.

The best values for samples containing 20% TiCN can be explained by the specific physical and mechanical properties of this two-phase compound, primarily determined by the nature of interatomic bonds depending on the ratio of carbon and nitrogen atoms [91]. Some nitrogen atoms are replaced by carbon atoms in titanium carbonitride, forming unlimited TiN-TiC solid solutions. Titanium nitride has the same crystal lattice as titanium carbide, and it is capable of replacing it isomorphically [92]. TiC*x*N*<sup>y</sup>* compounds [93,94] formed in the structure of CPHSS samples combine the advantages of carbide and nitride phases. The hardness of titanium carbonitride even slightly exceeds the value for titanium carbide when the plasticity is not inferior to that for titanium nitride (which is extremely important for the tool's operation during milling when the risk of chipping of the cutting edges increases). Titanium carbonitride inclusions have high thermodynamic stability and are closer to HSS in terms of thermal expansion coefficient. In addition, the improved performance for specimens with 20% TiCN compared to 20% TiC results from a lower affinity for iron-containing steels and alloys and a lower adhesive activity when heated [92–94]. The fact that the introduction of Al2O3 particles into a powder composition based on TiC somewhat improves the characteristics of CPHSS, but at the same time it is inferior to samples with TiCN additives, indicates the need for separate studies related to the optimization of the compositions of powder mixtures during sintering of experimental instrumental materials.
